With development of semiconductor technologies, process nodes (critical dimensions) become smaller and demands for materials become higher. For a conventional field effect transistor (FET), source and drain regions can have different doping types and concentrations from channel region, and PN junctions can be formed between the source and the channel or the drain and the channel. As the size of devices shrinks, effects such as shifts of threshold voltage and elevations of leakage current may affect device performance. In some cases, device structures such as silicon-on-insulator, dual-gate, triple-gate, and ring-gate have been used to suppress short-channel and other adverse effects.
Since the device has continually been scaled down in size, areas of source, drain, and channel regions become smaller, control of doping process becomes more challenging, and formation of PN junctions between source and channel or drain and channel becomes more difficult. To overcome problems including sudden change of doping type and concentration, junctionless transistor having source, drain and channel regions of the same doping type has been developed. Meanwhile, junctionless transistor can suppress short-channel effect, and can operate even when the critical dimension (CD) has been scaled down to a few nanometers.
A junctionless transistor having a homogeneous material as substrate may have high source-drain parasitic capacitance in regions surrounding source and drain, which can seriously affect performance of the junctionless transistor. A junctionless transistor having a silicon-on-insulator (SOI) substrate can have reduced parasitic capacitance between the source and the substrate or between the drain and the substrate. However, manufacturing cost of SOI substrate with small CDs can be very high. In addition, buried layers in the SOI substrate may have low thermal conductivity, thus heat generated in channel region may not be released as desired. This may cause overheating to affect the performance of the junctionless transistor.